Magnetic sensor integrated circuit with test conductor

ABSTRACT

A magnetic sensor integrated circuit includes a plurality of magnetically sensitive elements, and at least one test conductor positioned adjacent to at least one of the magnetically sensitive elements and configured to generate a differential magnetic field that is adapted to be applied to the plurality of magnetically sensitive elements during a test mode.

BACKGROUND

Some magnetic speed sensors are configured to measure the speed of amagnetic tooth wheel. Such speed sensors typically include an integratedcircuit with a plurality of magnetic sensor elements, such as Hallsensor elements or xMR sensor elements (e.g., GMR—giant magnetoresistance; AMR—anisotropic magneto resistance; TMR—tunnel magnetoresistance; CMR—colossal magneto resistance). A permanent magnetprovides a bias magnetic field to the sensor elements. As the wheel isrotated, the teeth of the wheel pass in front of the sensor and generatea small field variation, which is detected by the integrated circuit.The detected field contains information about the angular position androtational speed of the wheel.

It is desirable to be able to test magnetic sensors, such as magnetictooth wheel speed sensors, to help ensure that the sensors are operatingproperly. One method for testing a magnetic sensor is to use a magneticcore to apply test differential magnetic fields to the sensor, andmeasure the sensor response. One problem with using such an externalmagnetic field source is that there must be a precise alignment betweenthe magnetic core and the sensor under test. Position errors can resultin inaccurate test results.

SUMMARY

One embodiment provides a magnetic sensor integrated circuit. Theintegrated circuit includes a plurality of magnetically sensitiveelements, and at least one test conductor positioned adjacent to atleast one of the magnetically sensitive elements and configured togenerate a differential magnetic field that is adapted to be applied tothe plurality of magnetically sensitive elements during a test mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the present invention and are incorporated in andconstitute a part of this specification. The drawings illustrate theembodiments of the present invention and together with the descriptionserve to explain the principles of the invention. Other embodiments ofthe present invention and many of the intended advantages of the presentinvention will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 is a diagram illustrating a prior art speed sensor for sensingthe speed of a magnetic tooth wheel.

FIG. 2 is a diagram illustrating a top view of a magnetic sensorintegrated circuit according to one embodiment.

FIG. 3 is a diagram illustrating the magnetic sensor integrated circuitshown in FIG. 2 with the addition of test conductors that are positionedadjacent to the magnetically sensitive elements according to oneembodiment.

FIG. 4 is a diagram illustrating a top view of a magnetic sensorintegrated circuit according to another embodiment.

FIG. 5 is a diagram illustrating a cross-sectional view of a magneticsensor integrated circuit according to one embodiment.

FIG. 6 is a diagram illustrating a simplified cross-sectional view of amagnetic sensor integrated circuit according to one embodiment.

FIG. 7 is a diagram illustrating a top view of the magnetic sensorintegrated circuit shown in FIG. 6 according to one embodiment.

FIG. 8 is a flow diagram illustrating a method of testing a magneticsensor integrated circuit according to one embodiment.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

FIG. 1 is a diagram illustrating a prior art speed sensor 102 forsensing the speed of a magnetic tooth wheel 114. The speed sensor 102includes a permanent magnet 106 and a magnetic sensor integrated circuit110 surrounded by a protective cover 104. The magnetic sensor integratedcircuit 110 includes a plurality of magnetically sensitive elements 108,such as Hall sensor elements or xMR sensor elements (e.g., GMR—giantmagneto resistance; AMR—anisotropic magneto resistance; TMR—tunnelmagneto resistance; CMR—colossal magneto resistance). The permanentmagnet 106 provides a bias magnetic field to the elements 108. In theillustrated embodiment, the bias magnetic field is perpendicular to theplane of the integrated circuit 110 (e.g., in the Y-direction). Theelements 108 are separated from the magnetic tooth wheel 114 by an airgap distance 112. As the wheel 114 is rotated in the direction shown byarrow 116, the teeth of the wheel 114 pass in front of the sensor 102and generate a small field variation, which is detected by theintegrated circuit 110. The detected field contains information aboutthe angular position and rotational speed of the wheel 114. The waveformof the field is nearly sinusoidal and its amplitude decreasesdrastically with the air gap 112.

FIG. 2 is a diagram illustrating a top view of a magnetic sensorintegrated circuit 200 according to one embodiment. In one embodiment,magnetic sensor 200 is configured as a speed sensor, such as a speedsensor to sense the speed or position of a magnetic tooth wheel.Magnetic sensor 200 includes a semiconductor die 202, magneticallysensitive elements 204A-204D, interconnects 206A-206F, output terminals208, supply pad 210, and ground pad 212. The magnetically sensitiveelements 204A-204D are connected together and to pads 210 and 212 viainterconnects 206A-206F. Magnetic sensor 200 is configured as a fullbridge with two magnetically sensitive elements 204A-204B on the lefthand side of the die 202, and two elements 204C-204D on the right handside of the die 202. In another embodiment, magnetic sensor 200 isconfigured as a half bridge having two magnetically sensitive elements,with one element on the left hand side of the die 202, and one elementon the right hand side of the die 202. In yet another embodiment, die202 also includes an additional magnetically sensitive element at thecenter of the die 202 for direction measurement. In one embodiment, themagnetically sensitive elements 204A-204D are xMR elements.

In operation according to one embodiment, a voltage is applied to supplypad 210 causing a current to flow through the elements 204A-204D toground pad 212. In response to an external magnetic field, one or moreof the magnetically sensitive elements 204A-204D change in electricalresistance, causing a voltage signal at the output terminals 208. If adifferential magnetic field is applied to sensor 200, such that themagnetic field vector points to the right at the right half of the die202 and to the left at the left half of the die 202 (or vice versa), theoutput signal is positive or negative. Since magnetic sensor 200 isconfigured to process differential magnetic fields, the sensor 200 isinsensitive to homogenous magnetic fields.

It is desirable to be able to test magnetic sensors, such as sensor 102or sensor 200, to help ensure that the sensors are operating properly.One embodiment provides a magnetic sensor integrated circuit thatincludes at least one test conductor integrated on-chip to generatedifferential magnetic fields during a test mode of the chip. In oneembodiment, a homogeneous magnetic field that is generated off-chip isalso applied to the chip during the test mode. The magnetic fieldsapplied to the magnetic sensor during the test mode are used to test thefunctionality of the sensor. The system and method according to oneembodiment do not suffer from the position tolerance problems of priortesting techniques.

FIG. 3 is a diagram illustrating the magnetic sensor integrated circuitshown in FIG. 2 with the addition of test conductors 302A and 302B thatare positioned adjacent to the magnetically sensitive elements 204A-204Daccording to one embodiment. In the illustrated embodiment, magneticsensor 300 includes the same elements as magnetic sensor 200 (FIG. 2),but also includes test conductors 302A and 302B, interconnects304A-304C, and current-in pad 306. As shown in FIG. 3, test conductor302A is positioned underneath magnetically sensitive elements 204A and204B, and test conductor 302B is positioned underneath magneticallysensitive elements 204C and 204D. In another embodiment, test conductors302A and 302B are each two separate conductors, such that eachmagnetically sensitive element 204A-204D has a separate test conductorpositioned underneath it. In one embodiment, test conductors 302A and302B are positioned within 5 μm or less of the magnetically sensitiveelements 204A-204D, and run parallel with the magnetically sensitiveelements 204A-204D for at least about 25% of the length of themagnetically sensitive elements 204A-204D. In one specific embodiment,the test conductors 302A and 302B are positioned underneath themagnetically sensitive elements 204A-204D at a distance of about 600 nmaway from the elements 204A-204D. In another embodiment, the testconductors 302A and 302B are positioned above the magnetically sensitiveelements 204A-204D. The test conductors 302A and 302B are electricallyisolated from the magnetically sensitive elements 204A-204D by an oxidelayer in one embodiment. Test conductors 302A and 302B are connectedtogether via interconnect 304A. Test conductor 302A is connected tocurrent-in pad 306 via interconnect 304B. Test conductor 302B isconnected to ground pad 212 via interconnect 304C. In one embodiment,magnetic sensor 300 has a total chip area of less than about 10 mm.

In operation according to one embodiment, a voltage is applied to or acurrent is injected into current-in pad 306, causing a current to flowthrough the test conductors 302A and 302B to ground pad 212. In oneembodiment, conductor 302A generates a magnetic field that points to theleft for the left two elements 204A-204B and conductor 302B generates amagnetic field that points to the right for the right two elements204C-204D when a current flows through the conductors. In anotherembodiment, conductor 302A generates a magnetic field that points to theright for the left two elements 204A-204B and conductor 302B generates amagnetic field that points to the left for the right two elements204C-204D when a current flows through the conductors. Thus, the testconductors 302A and 302B generate a differential magnetic field on themagnetic sensor 300. In one embodiment, test conductors 302A and 302Bgenerate magnetic field components that are substantially parallel tothe surface or plane of the sensor 300 over the entire length orsubstantially the entire length of the magnetically sensitive elements204A-204D. The magnetic sensor 300 processes the signals generated bythe magnetically sensitive elements 204A-204D in the test mode in anormal manner (i.e., in the same manner that the signals are processedin a normal mode of operation), and generates corresponding outputsignals.

FIG. 4 is a diagram illustrating a top view of a magnetic sensorintegrated circuit 400 according to another embodiment. Magnetic sensor400 includes a semiconductor die 402, test conductors 404A-404D,interconnects 406A-406E, supply pad 408, transistor 410, ground pad 412,and controller 414. In one embodiment, magnetic sensor 400 also includesfour magnetically sensitive elements (not shown in FIG. 4), such as xMRelements, with one magnetically sensitive element being positioned overeach of the test conductors 404A-404D. The test conductors 404A-404D areconnected together, and to supply pad 408 and transistor 410, viainterconnects 406A-406E.

Test conductors 404A-404D are connected in series between supply pad 408and transistor 410. Transistor 410 connects the conductors 404A-404D toground pad 412. In one embodiment, transistor 410 is an NMOS FET, with adrain connected to interconnect 406E, a source connected to ground pad412, and a gate that is connected to a controller 414 within theintegrated circuit 400, which causes the transistor 410 to be turned onand off during the test mode. When transistor 410 is turned on, currentflows through the conductors 404A-404D, thereby generating adifferential magnetic field that is applied to the xMR elements.

In one embodiment, the drain of transistor 410 is also connected to atest pad or bond pad (not shown). In this embodiment, the currentthrough the conductors 404A-404D can be measured by connecting the testpad or bond pad via an ammeter to ground while the transistor 410 isoff. In one embodiment, transistor 410 is a high-voltage NMOS devicewith a drain enhancement (e.g., a larger distance between gate anddrain, so that there is a high voltage drop between gate and drainwithout excessive electric field), and with a width-to-length ratio(W/L) of about 100 to 5000. In another embodiment, the W/L ratio oftransistor 410 is about 1000 or higher. In one embodiment, transistor410 is implemented with two NMOS devices of equal size that areelectrically connected in parallel and that share a common source. Inanother embodiment, transistor 410 is a PMOS FET. However, due to thesmaller mobility of PMOS FETs, more space on the die is typicallyneeded.

In the embodiment shown in FIG. 4, transistor 410 is configured as asingle-transistor switch. In another embodiment, multiple transistors ina current mirror configuration are used in place of transistor 410. Acurrent mirror can generate a well-defined output current that is avariable portion or multiple of an input current. The current mirrorconfiguration is useful if the current and magnetic field are to bechanged (e.g., lowered to verify operation at a smallest possibleair-gap for an application).

In one embodiment, the magnetically sensitive elements on the left handside of the magnetic sensors 102, 200, 300, and 400 are separated fromthe magnetically sensitive elements on the right hand side of thesensors by a distance of greater than about 0.2 mm. In one specificembodiment, the spacing is about 2.5 mm. In one embodiment, themagnetically sensitive elements and the test conductors are each about1.4 mm long. In one embodiment, the integrated test conductors (e.g.,conductors 302A-302B and 404A-404D) are each 4 μm wide or narrower. Inone embodiment, the interconnects between the integrated test conductors(e.g., interconnects 304A and 406A, 406B, and 406D) are each about 2.5mm long and 20 μm wide.

In one embodiment, the supply pad 408 for the test conductor isidentical to a supply pad of functional parts of the integrated circuit400. In a specific embodiment, the supply pad 408 is identical to theoverall supply pad of the entire integrated circuit 400.

FIG. 5 is a diagram illustrating a cross-sectional view of a magneticsensor integrated circuit 500 according to one embodiment. Positioned atthe bottom of the magnetic sensor 500 is a silicon substrate layer 540.In one embodiment, transistor 410 (FIG. 4) is formed in substrate layer540. In the illustrated embodiment, magnetic sensor 500 includes threemetal layers or interconnect layers—metal-1 layer 536, metal-2 layer526, and GMR-metal layer 516. Each of the metal layers 516, 526, and 536includes a top and a bottom coating. Metal-1 layer 536 includes a topcoating 534 and a bottom coating 538. Metal-2 layer 526 includes a topcoating 524 and a bottom coating 528. GMR-metal layer 516 includes a topcoating 514 and a bottom coating 518. In one embodiment, each of thecoating layers is either titanium or titanium nitride (TiN), or acombination of titanium and TiN. The metal layers 516, 526, and 536 maybe connected together and to other elements in magnetic sensor 500 withvias, such as the vias 522 and 532 shown in FIG. 5. Oxide layers 512,520, and 530 are formed over the metal layers 516, 526, and 536,respectively, and provide electrical isolation between the metal layers.

FIG. 5 also shows an xMR element 510 formed on the oxide layer 512. Anoxide hard mask layer 508 is formed on the xMR element 510, and an oxidelayer 506 is formed over the mask layer 508 and the xMR element 510. Anitride layer 504 is formed on the oxide layer 506. A photoimide layer502 is formed on the nitride layer 504.

In one embodiment, the test conductors 302A-302B (FIG. 3) and 404A-404D(FIG. 4) are made from metal at the GMR-metal layer 516. The GMR-metallayer 516 is closest to the xMR element 510, and it is the thickest andhas the smallest sheet resistance of the three metal layers shown inFIG. 5. In one embodiment, the conductor interconnects (e.g.,interconnects 304A-304C and 406A-406E) are also made from metal at theGMR-metal layer 516. In another embodiment, the interconnects are madefrom metal at the metal-2 layer 526 or the metal-1 layer 536, and thewidth of the interconnects is increased in order to keep the resistanceconstant. In one embodiment, the test conductors 302A-302B and 404A-404Deach have a resistance of 20.5 Ohms, and each of the interconnects has aresistance of 7.3 Ohms. If the transistor 410 (FIG. 4) is positionedbelow the right magnetically sensitive elements (e.g., elements204C-204D), and is made large with an Rds on-resistance of less than 2Ohms for a 3V gate-drive, and if the drain and the source of thetransistor 410 have a contact resistance of 5 Ohms, then for a supplyvoltage of +3V, the resulting current through the test conductors in thetest mode is 25.9 mA (i.e., 3V/(4*20.5+3*7.3+5+2+5)=25.9 mA). Thiscurrent generates a 2.8 mT (milli-Tesla) magnetic field at themagnetically sensitive elements. The current is low enough to begenerated on-chip during the test mode, and the field is similar tothose applied in actual applications.

As mentioned above, the test conductors 302A-302B and 404A-404D are madefrom metal at the GMR-metal layer 516 in one embodiment. In anotherembodiment, additional test conductors are formed at the metal-2 layer526 below the test conductors at the GMR-metal layer 516, and theadditional test conductors are connected in series with the testconductors at the GMR-metal layer 516. In this embodiment, the overallresistance increases, the current decreases to 11 mA, and the magneticexcitation generated by the test conductors decreases to 2.0 mT. Inanother embodiment, rather than reducing the current with additionaltest conductors at multiple metal layers, the current is reduced bycontrolling the current with a current mirror in place of transistor410.

If the current through the integrated test conductors 302A-302B or404A-404D is generated on-chip, it is typically subject to processspreads. If this current is not accessible to a tester, it may not bepossible to calibrate the magnetic sensitivity of the sensor (i.e., theratio of the output of the sensor to the magnetic field magnitude) usingthis current. Since the on-chip generated current is not preciselyknown, the magnetic field resulting from this current is not preciselyknown. In one embodiment, the on-chip generated current is madeaccessible to a tester for measurement via a test pad (e.g., duringwafer level test). The on-chip generated current may also be trimmedduring a wafer level test so that it remains precise and well definedduring subsequent operation and tests (e.g., during a package leveltest).

FIG. 6 is a diagram illustrating a simplified cross-sectional view of amagnetic sensor integrated circuit 600 according to one embodiment.Magnetic sensor 600 includes a magnetically sensitive element 602 (e.g.,an xMR element), a test conductor 604, a metal-2 line 606, metal-1 lines608A-608C, contacts 610A-610C, NMOS transistor channels 612A and 612B,and oxide layer 614. As mentioned above, in one embodiment, transistor410 (FIG. 4) is implemented with two NMOS devices of equal size that areelectrically connected in parallel and that share a common source.Contacts 610A and 610C connect the two drains of the transistor 410 tometal-1 lines 608A and 608C, respectively. Contact 610B connects thesource of the transistor 410 to metal-1 line 608B. The positioning ofthe transistor 410 underneath the magnetically sensitive element 602results in good thermal coupling of the element 602 to the substrate.

FIG. 7 is a diagram illustrating a top view of the magnetic sensorintegrated circuit 600 shown in FIG. 6 according to one embodiment.During a test mode of the sensor 600, a current is applied to a supplypad 704. The arrows in FIG. 7 show how the current flows from the supplypad 704 through the magnetic sensor 600 to ground pad 702. The currentbegins at the supply pad 704 and flows upward through test conductor604. At the top of the conductor 604, the current branches left andright and then flows downward through metal-1 lines 608A and 608C. Themetal-1 lines 608A and 608C correspond to the two drains of thetransistor 410. The current from the left side drain (i.e., metal-1 line608A) flows rightward through channel 612A, and the current from rightside drain (i.e., metal-1 line 608C) flows leftward through channel612B. The current from the drains then enters the metal-1 line 608B,which corresponds to the source of the transistor 410. The current flowsupward through the metal-1 line 608B and into the ground pad 702.

In the illustrated embodiment, the current in the two metal-1 lines 608Aand 608C flows downward, which is the opposite direction of the currentthrough the test conductor 604. Thus, the current through the twometal-1 lines 608A and 608C reduces the magnetic field on themagnetically sensitive element 602. However, in one embodiment, the twometal-1 lines 608A and 608C are positioned sufficiently distant from theelement 602, and are made sufficiently broad (e.g., to reduce thecurrent density through these lines), so that their influence on theelement 602 is small. The current in the metal-1 line 608B is upward,which is the same direction as the current through the test conductor604. Thus, the current through the metal-1 line 608B generates amagnetic field on the element 602 that adds to the field generated bythe test conductor 604, which is beneficial.

FIG. 8 is a flow diagram illustrating a method 800 of testing a magneticsensor integrated circuit (e.g., magnet sensor 300 or 400) according toone embodiment. At 802, the magnetic sensor enters a test mode. At 804,a current is applied to at least one test conductor integrated into themagnetic sensor (e.g., test conductors 302A-302B or 404A-404D). In oneembodiment, the current is a pulsed current that causes the testconductors to produce a pulsed differential magnetic field onmagnetically sensitive elements within the magnetic sensor (e.g.,elements 204A-204D). In one embodiment, the current applied to the testconductors is in the range of about 0 to about 50 mA, and is pulsed atfrequencies in the range of about 5 Hz to about 50 kHz.

At 806, the magnetic sensor processes the signals produced by themagnetically sensitive elements, and generates output signals inresponse to the pulsed magnetic field. In one embodiment, the magneticsensor processes signals and generates output signals in the test modein the same manner as in a normal mode of operation. For example, theoutput signal of the magnetic sensor will go high if the magnetic fieldon the magnetically sensitive elements on the right side of the chip islarger than the field on the left side (e.g., Bx(right)−Bx(left)>0), andthe output signal of the magnetic sensor will go low if the magneticfield on the magnetically sensitive elements on the left side of thechip is larger than the field on the right side (e.g.,Bx(right)−Bx(left)<0).

At 808, the output signal generated by the magnetic sensor is monitoredto determine if the magnetic sensor is working properly and according tospecifications. In one embodiment, it is determined at 808 whether theoutput signal switches states at all (i.e., goes from high to low and/orfrom low to high). In one embodiment, the time delay between thetransition (i.e., rising or falling edge) of the pulsed current throughthe integrated test conductors and the corresponding transition of theoutput signal of the magnetic sensor is also determined at 808. It isstraightforward to measure the time when the pulsed current has atransition if this current is supplied to the magnetic sensor by anexternal source. If the current is generated on-chip, the transition canbe detected by monitoring the overall current consumption of themagnetic sensor.

In one embodiment, the magnetic sensitivity of the magnetic sensor isalso determined at 808, and it is determined whether the magneticsensitivity is within a proper range. For some magnetic sensorintegrated circuits, the signal processing circuitry of the magneticsensor extracts the amplitude of the signals generated by themagnetically sensitive elements. For such magnetic sensors, theamplitude of these signals and the amplitude of the current through theintegrated test conductors are used to determine the magneticsensitivity of the sensor.

In one embodiment, the phase-jitter of the output signal is alsodetermined at 808. This determination may be a little more difficult forcurrent pulses that are generated on-chip, since the current pulses mayexhibit significant jitter themselves (e.g., because the current pulsesare controlled by an on-chip oscillator in one embodiment, which couldhave a poor quality factor if no quartz is used). Nevertheless, thejitter of the output signal can be compared to the jitter of the currentpulses on the supply line. The difference of the two values is a measureof jitter performance of the magnetic sensor signal processing circuit.The jitter of the current pulses on the supply line is a measure of thequality of the on-chip oscillator. Thus, jitter-performance of theoscillator and the signal processing circuitry may be separatelydetermined.

Generally, the phase jitter of the output signal in response to externalfields will be much larger than the fields generated by the integratedtest conductors, because these external fields are asynchronous to theon-chip oscillator. A primary reason for jitter in the output signal isthat the sensor system samples the magnetic field asynchronously to itstransitions and thus may be delayed by up to half a period of the cyclefrequency. To this end, it may be advantageous to use an auxiliaryoscillator, which runs asynchronously to the main oscillator. Theauxiliary oscillator is started only during the test mode and serves togenerate asynchronous current pulses.

At 810, the amplitude of the pulsed current applied to the integratedtest conductors is varied. If the current is supplied to the integratedtest conductors via a test pad of the magnetic sensor during a waferlevel test, different currents are injected into the test pad at 810. Ifthe current is supplied via a bond pad after the magnetic sensor hasbeen packaged, different currents are injected into the bond pad at 810.If the current is generated on-chip, in one embodiment, the current iscontrolled by a signal that is provided via a pad on the magnetic sensoror by a digital code that is transferred to the magnetic sensor via adata interface protocol when entering the test mode. At 812, the lowestamplitude that makes the output signal switch is determined.

At 814, characteristics of the current through the integrated testconductors are varied, and one or more of the tests or determinationsperformed at 808 and 812 are repeated. In one embodiment, the polarity,amplitude, frequency, or duty-cycle of the current pulses applied to theintegrated test conductors are varied at 814. In one embodiment, thecurrent pulses are changed at 814 from bipolar current pulses tounipolar current pulses, or vice versa. In one embodiment, the magneticsensor is configured to selectively apply current pulses only toselected ones of the integrated test conductors in order to test thecharacteristics of individual ones of the magnetically sensitiveelements of the magnetic sensor.

At 816, a homogeneous magnetic field is applied to the entire magneticsensor at the same time that a pulsed differential magnetic field isbeing generated by the integrated test conductors. In one embodiment,the homogenous magnetic field is applied to the magnetic sensor byplacing the sensor in a large coil that generates the homogeneousmagnetic field. In one embodiment, the homogenous magnetic field isparallel to the plane of the integrated circuit (e.g., in thex-direction, which is the sensitive direction for GMRs). In otherembodiments, the homogenous magnetic field is applied to the integratedcircuit in the y-direction (e.g., perpendicular to the plane of theintegrated circuit) or in the z-direction (e.g., parallel to the planeof the integrated circuit and parallel to the current flow through theGMRs). When the field is applied in the y or z direction, theperformance of the integrated circuit will typically decrease (e.g., thehysteresis will typically increase), and such a field is useful indetermining the effects of a misalignment of the permanent magnet usedin actual implementations.

At 818, the magnitude and sign of the homogenous magnetic field isvaried. In one embodiment, the homogenous magnetic field is rampedupward at 818 from −20 mT to +20 mT. At 820, the saturation point of themagnetic sensor is determined. The repetitive pulsed current applied tothe integrated test conductors produces a pulsed differential magneticfield, which makes the output signal of the magnetic sensor switch. Ifthe magnetic sensor saturates at, for example, +/−10 mT, the outputsignal of the magnetic sensor will stop switching when the magnitude ofthe homogenous magnetic field exceeds +/−10 mT.

At 822, the degree of linearity of the magnetic sensor output signalversus magnetic field strength is determined. The degree of linearitymay be determined by comparing the measured amplitudes of the magneticsensor output signal to the magnitudes of the homogenous magnetic field.

At 824, hysteresis characteristics of the magnetic sensor aredetermined. The hysteresis characteristics may be determined bymonitoring the sensor output signal while the homogenous magnetic fieldis ramped up to a maximum value, and then ramped down to zero, andpreferably ramped down to a maximum negative value, and then ramped upto zero again.

At 826, it is determined whether the magnetic sensor is properlydetecting direction of movement. Typically, direction detection isaccomplished by the use of an additional magnetically sensitive elementthat is placed in the center of the die (e.g., a direction xMR element).In one embodiment, an additional test conductor is positioned below thedirection xMR element to separately produce its own test magnetic fieldindependent of the magnetic fields produced by the other testconductors. In another embodiment, a magnetic field is applied to thedirection xMR element via an external source, such as the homogenousmagnetic field applied at 816. If the amplitude of the applied magneticsignal is small compared to the saturation point of the magnetic sensor(e.g., only a few mT), the small magnetic field will add linearly to thesmall fields of the outer integrated test conductors (i.e., theintegrated conductors positioned below the speed xMR elements). If themagnetic sensor is a first order gradiometer, its output will notrespond to this homogeneous background field. Therefore, the homogeneousbackground field acts only on the direction xMR element, whereas thefield generated by the on-chip test conductors acts only on the speedxMR elements. Thus, the direction xMR element and the speed xMR elementscan be tested independently.

In order to simulate operation in the field, the direction field (e.g.,the homogenous magnetic field applied to the magnetic sensor) should beout of phase with the speed-field (i.e., the magnetic field applied tothe speed xMR elements by the integrated conductors). If the speed fieldis generated on-chip, the phase of the speed field can be detected bymonitoring the current pulses on the supply line of the chip. The phasedifference between the speed field and the direction field, as well asthe amplitude, frequency, and duty-cycle of both fields can be varied todetermine if the magnetic sensor is properly detecting direction ofmovement.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A magnetic sensor integrated circuit, comprising: a plurality ofmagnetically sensitive elements; at least one test conductor positionedwithin the integrated circuit adjacent to at least one of themagnetically sensitive elements and configured to conduct a current thatgenerates a differential magnetic field that is adapted to be applied tothe plurality of magnetically sensitive elements during a test mode,wherein the differential magnetic field points in a first direction fora first set of the magnetically sensitive elements and points in asecond direction opposite to the first direction for a second set of themagnetically sensitive elements; and a circuit for varying an amplitudeof the current through the at least one test conductor and determining alowest amplitude of the current that makes an output signal switchstates.
 2. The integrated circuit of claim 1, wherein the magneticallysensitive elements are one of giant magneto resistance (GMR) elements,anisotropic magneto resistance (AMR) elements, tunnel magneto resistance(TMR) elements, or colossal magneto resistance (CMR) elements.
 3. Theintegrated circuit of claim 1, wherein the plurality of magneticallysensitive elements comprises at least four magnetically sensitiveelements in a full bridge configuration.
 4. The integrated circuit ofclaim 3, wherein the at least one test conductor comprises two testconductors, and wherein a first one of the test conductors is positionedunder a first two of the magnetically sensitive elements, and wherein asecond one of the test conductors is positioned under a second two ofthe magnetically sensitive elements.
 5. The integrated circuit of claim1, wherein the plurality of magnetically sensitive elements comprises atleast two magnetically sensitive elements in a half bridgeconfiguration.
 6. The integrated circuit of claim 1, wherein the atleast one test conductor is positioned parallel to the at least onemagnetically sensitive element, and wherein the at least one testconductor is at least about twenty-five percent as long as the at leastone magnetically sensitive element.
 7. The integrated circuit of claim6, wherein the at least one test conductor is positioned within about 5μm of the at least one magnetically sensitive element.
 8. The integratedcircuit of claim 1, wherein the integrated circuit has a surface area ofless than about 10 mm².
 9. The integrated circuit of claim 1, whereinthe integrated circuit includes a plurality of interconnect layers, andwherein the at least one test conductor is formed at an interconnectlayer that is closest to the at least one magnetically sensitiveelement.
 10. The integrated circuit of claim 1, wherein the integratedcircuit includes a pad coupled to the at least one conductor forsupplying a current to the at least one test conductor from an externalsource.
 11. The integrated circuit of claim 1, wherein the at least onetest conductor has a width that is less than or equal to about 4 μm. 12.A method of testing a magnetic sensor integrated circuit, comprising:providing a magnetic sensor integrated circuit having a plurality ofmagnetically sensitive elements and at least one test conductorpositioned within the integrated circuit adjacent to at least one of themagnetically sensitive elements; causing the integrated circuit to entera test mode; generating a current through the at least one testconductor during the test mode, thereby generating a differentialmagnetic field that is adapted to be applied to the plurality ofmagnetically sensitive elements; monitoring an output signal of theintegrated circuit while in the test mode; varying an amplitude of thecurrent through the at least one test conductor; and determining alowest amplitude of the current that makes the output signal switchstates.
 13. A method of testing a magnetic sensor integrated circuit,comprising: providing a magnetic sensor integrated circuit having aplurality of magnetically sensitive elements and at least one testconductor positioned within the integrated circuit adjacent to at leastone of the magnetically sensitive elements; causing the integratedcircuit to enter a test mode; generating a current through the at leastone test conductor during the test mode, thereby generating adifferential magnetic field that is adapted to be applied to theplurality of magnetically sensitive elements; monitoring an outputsignal of the integrated circuit while in the test mode; and applying ahomogeneous magnetic field to the magnetic sensor simultaneously withthe differential magnetic field.
 14. The method of claim 13, and furthercomprising: determining a saturation point of the integrated circuit.15. The method of claim 13, and further comprising: determining a degreeof linearity of the output signal.
 16. The method of claim 13, andfurther comprising: determining hysteresis characteristics of theintegrated circuit.